Not Applicable
Not Applicable
This invention relates generally to optical fiber communication systems, and in particular to a splittable branch connector and a method for installing the branch connector in an existing ducting system in which channelization guide tubes and/or cables already have been installed.
Communication systems employing optical fibers have termination points where optical fiber cross connections, interconnections and terminations are established. The termination points are generally located at a customer""s premises, remote from a central office. To reach each termination point, the optical fiber cables must be separated (to form a branch) from a bundle of cables (a xe2x80x9ctrunkxe2x80x9d or xe2x80x9cringxe2x80x9d) and then are routed through a protective branch duct from various junctions or branch locations to remote customer interface access stations.
In the access network, the connection from the central exchange office to the customer passes many splices and branches. Splicing and branching in copper twisted-pair has been done for more than a century. However, for optical fiber (where a minimum of splices is preferred) the conventional technology does not suffice. Optical access networks require a high degree of versatility: it is not known when or where a connection will be requested, installation must be fast, new connections should not disturb existing ones, both business and consumer markets may ask for solutions requiring different quality of service, bandwidth provisions and redundant connections. Also excessive digging should be avoided and trench space is limited.
Other factors also must be considered when branching is performed in a fiber optic access network. A major concern is avoidance of damage to the cable during installation. Damage can occur in a variety of ways, namely 1) radial crushing the cable with installation equipment; 2) severe bending, twisting, flexing or stretching damage due to excessive forces applied during installation; 3) damaging the protective cable jacket, such as by abrasion, cracking or cutting the outer protective layer; and 4) long term exposure of the cable to environmental factors which cause thermal cycling.
Another concern is ease of installation and reduction in the amount of time needed to install the cable. Also, there is a concern to avoid splices in the cable as much as possible. Splices are time consuming to make and incur considerable installation costs. Generally, it is desirable to install the longest continuous length of cable possible to reduce the number of splices needed for the desired cable run. Moreover, it is not desirable to have a large number of splice joints in view of the relatively substantial signal damping caused by each joint in proportion to the total signal damping of the overall signal path.
Protective cable ducts have been channelized in an effort to satisfy these concerns. For this purpose a tube, whose interior may have a lower coefficient of friction than the existing duct, is installed in the existing protective duct, (direct buried is also possible), thereby establishing a separate channel in which cable, optionally at a later time, can be blown or pulled through the protective duct over a greater length. It may also be desirable to install in an existing duct a larger number of tubes with a smaller cross section than that of the existing duct if it is desired to use each of the smaller tubes as a separate channel or sub-duct for single- or multi-core copper or glass fiber cables. Further, it may be necessary to install in an existing duct a protective tube with a water barrier, so that in the existing duct, whose interior gradually fills up with water through diffusion, a waterproof conduit is created by means of the second tube, this waterproof conduit allowing the routing of cables without a water shield.
U.S. Pat. Nos. 4,850,569 and 4,934,662 to Griffioen et al. describe combining high speed air flow with a pushing force applied at the entry end of the conduit to install a traditional (i.e. with non-negligible stiffness) cable. These techniques also have been used advantageously for installing channelizing guide tubes in an existing protective duct. For example, in U.S. Pat. No. 5,884,384 to Griffioen, channelization is achieved by installing a bundle of guide tubes or sub-ducts in an existing protective duct by means a fluid under pressure, for instance compressed air, together with a pushing force exerted on the guide tubes as they enter the protective duct.
In U.S. Pat. No. 5,971,035 to Griffioen a method is provided for installing a ducting system with branches, wherein at the point of a branch in an existing duct of the system a tubular branch element with an inlet opening, an outlet opening and at least one branch opening is arranged by removing a duct portion from the existing duct at the point of the branch, by sliding the branch element on one of the free ends of the existing duct resulting from the interruption, replacing the removed duct portion or a portion identical in shape in the interruption and moving and securing the branch element in such a manner that the inlet opening and the outlet opening engage in sealing manner over the respective ends of the existing duct. In the duct with branches, over the entire length thereof, a bundle of sub-ducts can be arranged, which sub-ducts can then be branched off in a simple manner at the location of a branch. In a duct thus branched-off, for instance a continuous fiber optic cable can be installed.
The method and branch apparatus of Griffioen U.S. Pat. No. 5,971,035 have achieved excellent results for new installations where branch locations are known in advance and the branch connections can be installed at the known locations. A limitation on this method is that such branching is intended for installation before cables are laid in, since it is necessary to cut and completely separate the free ends of the protective duct to allow serial attachment of the branch connector and end couplings. This means that any existing plant components, e.g. guide tubes and/or cables already laid, would also have to be cut and separated to allow installation of the branch connector. It would then be necessary to splice and restore the existing fiber cables (which would require some over-length provision and two splices to restore each fiber path), followed by waterproofing and mechanical restoration of the duct. This naturally would cause an interruption of existing cable services, also causing some signal loss and degradation at each splice point. For these reasons such interference with existing plant equipment is to be avoided as much as possible.
It will be appreciated that building optical access networks with conventional methods and equipment is challenged by the uncertainties imposed by growing demand. For example, to splice a branch-cable to a feeder cable it is required to build over-length (window cut) in the feeder cable, in order to allow splicing above the trench. This is done at a predetermined fixed branch position, close to the customer. If all next customers were known in advance, over-length and branches could also be built close to them. But this is hardly ever the case. The location where branches and over-length may be needed is just a guess. In practice new customers are far away from these locations. To avoid digging again along the feeder route extra tubes are laid parallel. A lot of trench space is consumed and much money is invested in outside plant. Also the number of fibers installed from the beginning means high initial costs. Moreover, to avoid numerous splices for every length extension more length must be installed than needed for the first customer.
Consequently, there is a continuing need for improvements in outside plant equipment and installation methods that can provide versatility to meet growing, unpredictable demand, reduce the number of splices required, and provide mid-span branching access at any place, any time, even after cables have been laid in existing protective ducts.
The invention provides a solution that makes it possible to install branches in an existing duct in which channelization guide tubes and/or cables have already been laid wherein the existing duct can be cut at any point where a branch is desired, without disturbing existing services. This is accomplished by a xe2x80x9csplittablexe2x80x9d branch connector that is mountable in an operative branching position on the free end portions of an existing duct that has been cut at any mid-span location where a branch is desired.
The branch connector includes a pair of separable housing members that are engagable with each other, each having end couplings for clamping onto the cut ends of the duct. The housing members include duct sidewall portions that can be installed in spaced relation around existing guide tubes and cables, thus forming a serial extension of the existing duct. When clamped together, the sidewall portions form a protective duct restoration passage through which the existing guide tubes and cables extend, undisturbed. The restoration housing members, when clamped together in the protective restoration position, also form an inlet opening and an outlet opening for receiving the cut free end portions of the main duct. Separable fasteners releasably hold the housing members in clamping engagement with each other and with the free end portions of the protective duct.
Either one or both of the restoration housing members include sidewall portions forming a tubular branch stub in communication with the restoration duct passage. According to this arrangement, the splittable branch connector restores the existing protective duct and provides a protective branch stub permitting serial connection of an existing guide tube to a branch guide tube through which a continuous drop cable can be installed by pushing and/or blowing.
According to an important aspect of the invention, best results are obtained when the guide tubes have been installed in a loose bundle that does not entirely fill up the protective duct to offer sufficient mechanical protection, to make all tubes accessible at any place and for ease of installation of the bundle. By this arrangement, each guide tube is readily available and easily maneuverable at any desired mid-span location. A short segment of the protective duct is cut away and removed, thus exposing the loose bundle of guide tubes. A selected (empty) guide tube is then cut, and the cut-free end is routed through the branch stub for serial attachment to a drop branch guide tube. The other cut end of the selected guide tube is sealed. After connecting the drop branch guide tube, a protective branch duct is attached to the branch stub by a screwable connector, and a cable is then installed through the branch by blowing and/or pushing.
Through the method of the present invention, at any desired mid-span point in the main duct, even after as well as before the installation of a bundle of guide tubes and/or cables, a branching point can be realized, whereby, due to the restoration of the continuous inside wall of the existing duct at the interruption, a continuous conduit for protecting the existing guide tubes and cables is maintained, while simultaneous a branching stub conduit is provided for routing a drop cable, all without damaging or disturbing the existing guide tubes and/or cables.
The invention also provides an improved branch connector for use in practicing the method of the present invention, characterized by separable, complementary housing portions, each in the form of a concave shell, and when assembled together forming an internal duct passage, an inlet opening, an outlet opening and at least one branch opening, with the housing portions forming the inlet opening and the outlet opening also including coupling means for connecting them in sealing engagement onto the spaced-apart cut end portions of the existing duct, and the housing portions forming the branch opening also including coupling means for connecting them in sealing engagement with a protective branch duct.